Messages in this thread | | | Date | Wed, 24 Mar 2010 19:09:12 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH 0/6] sched/cpusets fixes, more changes are needed |
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On 03/24, Peter Zijlstra wrote: > > On Mon, 2010-03-15 at 10:09 +0100, Oleg Nesterov wrote: > > > > - do_fork() clears PF_STARTING and then calls wake_up_new_task() > > which finally does s/WAKING/RUNNING. > > > > But. Nobody can take rq->lock in between. This means a signal > > from irq (quite possible with CLONE_THREAD) or another rt > > thread which preempts us can lockup. > > Hmm, the signal case might indeed be a problem, however I cannot see how > the RT thread can be a problem because until we do wake_up_new_task() > the child will not be runnable and can thus not be preempted.
Indeed, but I meant the _parent_ can be preempted ;)
In short. TASK_WAKING acts as a spinlock in fact. And since ttwu() can be called from any context, it should be irq-safe: any owner must disable inerrupts and preemption.
> The reason we have that TASK_WAKING stuff for fork is because > wake_up_new_task() needs p->cpus_allowed to be stable
Sure! But it is very easy to change wake_up_new_task() to set TASK_WAKING like ttwu() does. Of course, this needs raw_spin_lock_irq(rq->lock) for a moment, but afaics that is all?
> So the below patch makes select_task_rq_fair unlock the rq when needed, > and then puts all ->select_task_rq() calls under rq->lock. This should > allow us to remove the TASK_WAKING thing from fork which in turn allows > us to remove the PF_STARTING check in task_is_waking. > > How does that look?
I'll try to read this patch tomorrow. But could you please consider the suggestion above?
Oleg.
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